J. Phys. Chem. C 2008, 112, 15281–15284
15281
Phase Evolution and Morphology Control of ZnS in a Solvothermal System with a Single Precursor Linfeng Jiang, Ming Yang, Shiyao Zhu, Guangsheng Pang,* and Shouhua Feng State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: May 28, 2008; ReVised Manuscript ReceiVed: July 28, 2008
A series of ZnS nanostructures with controllable crystal phase and morphology were synthesized by a solvothermal method using Zn(NCS)2(C5H5N)2 as a precursor. Nanospheres, microflowers, and microspheres assembled with hexagonal or cubic nanocrystals were obtained in different solvents. Phase evolution of ZnS in polyols was also discussed. Hexagonal ZnS is stable in polyols with neighboring hydroxyl, whereas cubic ZnS is stable in polyols without neighboring hydroxyl. The ligation of hydroxyl groups with Zn atoms on the surface of ZnS is responsible for the phase evolution of ZnS in polyols. Introduction As an important material with a band gap energy of 3.6 eV, ZnS has been widely studied, and recently, special attention has been devoted to its complex structures, especially threedimensional (3D) architectures.1-3 These structures may provide opportunities to exploit novel properties because of their high surface area and surface permeability.4,5 ZnS nanostructural assemblies, including solid and hollow nanospheres,6 hierarchical structures,7 and nanowire/nanobelt arrays,8 were reported due to their superior characteristics. A series of ZnS nanostructures with various morphologies and sizes were obtained by a facile solvothermal method. For examples, quantum-sized ZnS nanocrystals with quasi-spherical and rod shapes were synthesized using alkylamine,9 bundles of wurtzite ZnS nanowires were synthesized using hydrazine hydrate,10 ethylenediamine was employed to prepare two-dimensional wurtzite ZnS nanostructures,11 spherical nanostructures assembled with ZnS nanocrystals were obtained in aqueous solutions,12 and ZnS nanoparticles and nanorods with controllable crystal structure were fabricated through a solvothermal approach by changing the solvent used for the synthesis.13 ZnS has two crystallographic structures of hexagonal wurtzite and cubic sphalerite. Hexagonal ZnS is a thermodynamically metastable phase, which is stable at temperatures higher than 1023 °C,14 whereas cubic sphalerite ZnS is a thermodynamically stable phase at ambient temperature. The hexagonal structure could be transformed spontaneously to the cubic by contacting with some organic molecules at ambient temperature.15 A transformation from hexagonal to cubic structure was observed when ZnS nanorods were doped with manganese ions.16 By solvothermal reaction, the thermodynamically metastable phase, hexagonal ZnS nanocrystals, was successfully produced at a relatively low temperature.12,17 Molecular dynamics simulations combined with thermodynamic analysis and experiments indicate that smaller ZnS nanoparticles in a vacuum are more thermodynamically stable in wurtzite phase than sphalerite phase.18 In our previous work, ZnO can be obtained by hydrothermally treating Zn(NCS)2(C5H5N)2 under the alkaline aqueous condi* To whom correspondence should be addressed. Tel.: +86-043185168317. Fax: +86-0431-85168624. E-mail:
[email protected].
tion.19 Here, we report the synthesis of phase- and shapecontrollable ZnS nano- or microstructures with Zn(NCS)2(C5H5N)2 as precursor via a one-step solvothermal approach by changing solvents and other experimental parameters. The phase evolution of ZnS in alkanols is also discussed. Experimental Section Zn(NCS)2(C5H5N)2 was prepared by the same method as described in ref 19. In a typical procedure, 14.9 g of zinc nitrate hexahydrate was dissolved in 200 mL of distilled water, and 8 g of ammonium thiocyanate was dissolved in 100 mL of distilled water. Then the two solutions were mixed together. A 20% pyridine aqueous solution was added dropwise into the above solution in an ice bath under stirring. The products were filtered and washed with ethanol four times and then dried at 70 °C. Solvothermal reactions were performed by adding a certain amount of Zn(NCS)2(C5H5N)2 into a 90 mL Teflon-lined autoclave, then filled with 72 mL of solvent. Ethylene glycol (EG), ethylenediamine (EN), and 1-hexanol were used as solvents for solvothermal treatment. The detailed experimental conditions are listed in Table 1. The autoclave was sealed, maintained at a certain temperature for hours, and then cooled to room temperature naturally. The products were centrifuged and washed with alcohol and distilled water, respectively, for several times, and finally dried at 60 °C. When EN was employed as the solvent at 220 °C, the autoclave was quenched to room temperature by water. The X-ray diffraction patterns (XRD) of the products were recorded on Rigaku D/MAX 2500/PC diffractometer with graphite-filtered Cu KR radiation. Scanning electron microscopy (SEM) was performed on a JSM-6700F electron microscope. Transmission electron microscopy (TEM) was performed on a JEM-3010 electron microscope. Results and Discussion Synthesis of Hexagonal ZnS Nanospheres in EG. The XRD pattern of sample A1 is shown in Figure 1a. All of the diffraction peaks were indexed to hexagonal ZnS (JCPDS 80-0007). The diffraction peaks were significantly broadened because of the small crystallite size, and the average crystallite size estimated by XRD line broadening is ca. 4.5 nm.
10.1021/jp804705v CCC: $40.75 2008 American Chemical Society Published on Web 09/05/2008
15282 J. Phys. Chem. C, Vol. 112, No. 39, 2008
Jiang et al.
TABLE 1: Experimental Conditions for the Synthesis of ZnS Samples A B C D
solvent
temp/°C
precursor/g
time/h
sample
ethylene glycol ethylenediamine mixturea 1-hexanol
220, 160, 180, 200 220 220 220
0.05 0.28 0.28 0.28
12 4 12 12
A1, A2, A3, A4 B1 C1-C5 D1
crystal phase hexagonal hexagonal hexagonal cubic
a The mixture was prepared by adding 2 M aqueous HNO3 solution into EN. The amount of HNO3 aqueous solution in the mixture was 0.1, 0.3, 0.6, 0.8, and 1.2 mL for C1, C2, C3, C4, and C5, respectively.
Figure 1. XRD patterns of ZnS samples: (a) A1; (b) B1; (c) D1.
TABLE 2: Detailed Morphology Evolution of Samples C1-C5
of Figure 2b-d, respectively. The formation of ZnS spheres can be attributed to the oriented aggregation of the initially formed ZnS nanocrystals.6 We also observed the lattice continuity among the attached crystals, which is shown in the HRTEM image (inset of Figure 2a). Synthesis of Hexagonal ZnS Microflowers in EN. Figure 1b shows the XRD pattern for sample B1, which matches well with hexagonal ZnS (JCPDS 36-1450). The narrower (002) peak in Figure 1b indicated that the nanocrystals were elongated along the c-axis. The SEM image in Figure 3a shows the microflowers of sample B1 with diameters in the range of 1-3 µm. A typical microflower is shown in Figure 3b. It should be noted that the pure hexagonal ZnS is only obtained when the autoclave was quenched to room temperature by water after the solvothermal reaction. If the autoclave was cooled to room temperature naturally, a mixture of hexagonal ZnS and lamella complex ZnS · 0.5EN can be obtained (see the Supporting Information). Previous results indicate that the complex ZnS · 0.5EN could be formed in pure EN at 180 °C, and hexagonal ZnS was obtained by annealing ZnS · 0.5EN in vacuum or an argon stream.20,21 Cubic ZnS is obtained in pure EN at 220 °C for a reaction time of more than 10 h.22 Hexagonal ZnS can be obtained in mixed solvents of water and ethanolamine.12 Our results suggest that the cooling speed is crucial in the formation of pure hexagonal ZnS via a one-step solvothermal treatment in EN. The lamella complex ZnS · 0.5EN appeared as a metastable phase, which is decomposed into the hexagonal ZnS at 200 °C in pure EN (see the Supporting Information). The following reversible reaction happens at 220 °C.
ZnS · 0.5EN ) ZnS + 0.5EN
Figure 2a shows the TEM image of sample A1. The quasispheres with diameter of ca. 80 nm are well-dispersed and uniform. The inset of Figure 2a is the high-resolution TEM (HRTEM) image on the edge of a quasi-sphere. The size of the nanocrystallites is ca. 4 nm, which is consistent with the size estimated by the XRD line broadening. The lattice fringes with an interplanar spacing of 0.31 nm match well with the (002) plane separation of cubic ZnS. Figure 2b-d shows the SEM images of samples A2, A3, and A4, which are prepared at 160, 180, and 200 °C, respectively. The reaction temperature has a crucial influence on the sizes of agglomerated ZnS spheres, and the diameters of ZnS spheres change greatly above 200 °C. The corresponding diameters of the spheres of A2, A3, and A4 are estimated to be ca. 200, 350, and 450 nm, as shown in the insets
(1)
If the autoclave was quenched to room temperature by water, the reversible reaction was inhibited, and pure hexagonal ZnS could be obtained. A small amount of 2 M HNO3 aqueous solution was introduced as an additive in the EN solvothermal system. The experiment details and results are provided in Table 2. The morphology of the products changed from microflowers to nanorods, and further to grains, with increasing the amount of HNO3 solution. The microflowers of sample C1 are assembled with nanorods, whose ends are thinner than those in pure EN. The SEM image of sample C2 shows radiate bundle structures which can be described as fragments of the microflowers. And some well-dispersed one-dimensional nanostructures are found in sample C3, as shown in the SEM/TEM images. The SEM/ TEM images of sample C4 show interlinked nanorods structure with diameters of 70-140 nm. The corresponding ED pattern in the inset indicates that the nanorods are elongated along the c-axis and aligned in the same orientation. Sample C5 consists of grains, as shown in the SEM/TEM images. All the selected area electron diffraction (SAED) patterns of samples C1-C5 (the insets of the corresponding TEM images) can be well indexed to be hexagonal ZnS. Phase Evolution in Alkanols. It has been known that the polyol plays a key role in forming hexagonal ZnS nanocrystals
Controllable Synthesis of ZnS
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15283
Figure 2. (a) TEM micrographs of A1 (inset: HRTEM image on the edge of a sphere). (b-d) SEM images of (b) A2, (c) A3, and (d) A4. The insets of panels b, c, and d are typical spheres of the corresponding sample.
Figure 3. (a) SEM images of B1. (b) High-magnification image of B1.
at low temperatures, and the polyol probably forms some intermediates with ZnS, which can decomposes into wurtzite ZnS at lower temperatures.17 We found that hexagonal and cubic phases of ZnS are formed in EG and 1-hexanol, respectively. All diffraction peaks of D1 shown in Figure 1c could be indexed to the cubic phase (JCPDS 80-0020). No evidence of the formation of hexagonal ZnS was found. The sample D1 prepared in 1-hexanol consists of well-dispersed microspheres with diameters of 1.5-3.5 µm (see the Supporting Information). In order to understand the influence of the polyol on the phase stability of ZnS in solvothermal system, we tried more solvents (1,6-hexdianol and 1,2-hexdianol), which have the similar functional OH groups. By replacing 1-hexanol with the above solvents and maintaining the other experimental parameters, hexagonal ZnS can only be formed in 1,2-hexdianol, as shown in Figure 4. The crystal structure of ZnS nanocrystals was sensitive to some organic molecules which acted as a surfacemodifying reagent.15 In the solvothermal system, the solvent also serves as a surface-modifying reagent. It was proposed that OH groups might lead to a change of the surface energy of ZnS nanocrystals by forming chemical bonds between the surface Zn atoms of ZnS nanocrystals and the OH groups, which may be responsible for the formation of the hexagonal ZnS nanocrystals, as reported previously.12 The hexagonal ZnS cannot be formed in alkanols with one OH group, such as ethanol, 1-hexanol, and benzyl alcohol, or in polyols without
Figure 4. XRD patterns of different ZnS samples prepared in (a) 1,6hexdianol and (b) 1,2-hexdianol.
neighboring OH groups, such as 1,6-hexdianol. The hexagonal ZnS can only be formed in polyols with neighboring OH groups, such as 1,2-hexdianol, EG, and glycerol (see the Supporting Information). The OH group may bond with zinc atoms on the surface of ZnS. The formation of ligation between the surface zinc atoms of ZnS and the OH groups of solvent may lead to a change of the surface energy of ZnS nanocrystals. The ligation of different polyols may be responsible for the formation of hexagonal or cubic ZnS. Hexagonal ZnS is stable in polyols with neighboring hydroxyls, which implies that the neighboring hydroxyls bond with neighboring Zn atoms on the surface of ZnS (Scheme 1). This results in lower surface energy. Whereas in polyols without neighboring hydroxyls or in alkanols with monohydroxyl, there
15284 J. Phys. Chem. C, Vol. 112, No. 39, 2008 SCHEME 1: Hydroxyl Groups of 1,2-Hexdianol and 1,6-Hexdianol Bond with Zn Atoms on the Surface of ZnS
Jiang et al. Supporting Information Available: XRD patterns of ZnS samples prepared in EN via the naturally cooling process or quenched process (Figure S1), XRD patterns of samples prepared in EN at 200 °C for different reaction times (Figure S2), SEM images of samples prepared in EN at 200 °C for different reaction times (Figure S3), XRD patterns of ZnS samples obtained in different solvents (Figures S4 and S6), SEM images of sample D1 (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
are two cases: one is only a single hydroxyl bonding with the Zn atom; the other is two hydroxyls bonding with neighboring Zn atoms. The Zn atoms on the surface of ZnS can not be “fixed”, because of the flexibility of long carbon chains in the polyol molecule (Scheme 1). In these cases, the surface of ZnS has higher surface energy, and the cubic ZnS is stable. The analysis regarding the surface energy is consistent with the results obtained from dynamic simulations of free faces of ZnS crystals at 300 K, which indicates that the average surface energy of sphalerite phase ZnS is higher than that of wurtzite phases.18 Conclusions In summary, we have demonstrated a simple solvothermal method for the synthesis of various ZnS nano- and microstructures assembled by ZnS nanocrystals with controllable crystal phases. Hexagonal ZnS nanospheres with different diameters were formed in EG. Microflowers assembled with onedimensional nanostructures were obtained in EN. Cubic ZnS microspheres assembled with nanocrystals were formed in hexyl alcohol. Phase evolution of ZnS in polyols was also discussed. Hexagonal ZnS is stable in polyols with neighboring hydroxyl, whereas cubic ZnS is stable in polyols without neighboring hydroxyl. The ligation of hydroxyl groups with Zn atoms on the surface of ZnS is responsible for the phase evolution of ZnS in polyols. Acknowledgment. This research was supported by the National Natural Science Foundation of China (Grant Nos. 20671039 and 20121103).
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